The current invention relates to arbitrary waveform generators for playing back stored waveforms, and more particularly to phase coherent playbacks of stored waveforms when switching between different stored waveforms.
Arbitrary waveform generators (AWG), such as the AWG7000 series manufactured by Tektronix, Inc. of Beaverton, Oreg., use a digital-to-analog (D/A) converter to play out waveforms that are stored digitally in a memory. The AWG provides various methods for switching playout on and off, and for switching between waveforms.
FIG. 1 shows a block diagram view of a portion of an AWG according to the prior art. A waveform memory stores information for a plurality of waveforms. For each waveform the waveform memory stores an ordered set of digitized samples that correspond to that particular waveform. A waveform playout system reads portions of the ordered sets of digitized samples from the waveform memory over a memory access connection. The waveform playout system uses techniques, which are well known to those skilled in the art, for reading efficiently from the waveform memory, such as reading several digitized samples in a single parallel operation. The waveform memory is organized in a special way to support such efficient reading, as is well known in the art.
The waveform playout system produces an output analog voltage from a D/A converter on a waveform output line by repeatedly converting the digitized samples into output analog voltage levels. The rate of this conversion is given by an output sample rate, R, which is a number measured in samples per second, and which is a parameter of operation of the waveform playout system. The parameter, R, typically may be altered by a user of the AWG. The rate at which the waveform playout system uses the memory access connection to access the waveform memory is not necessarily the same as R. All that is required is that, on average, the waveform playout system obtains digitized samples from the memory at a higher rate than the rate needed to produce the waveform output, as is well known in the art.
The operation of the waveform playout system is further governed by a sequencer, as described below. FIG. 2 shows a timing diagram for the AWG of FIG. 1 to illustrate a typical operation of the AWG. The sequencer interacts with the waveform playout system by activating a step indication at a moment in time, t0. At time, t0, the sequencer also sends to the waveform playout system two additional items of information: (1) a waveform selection which selects the waveform to begin playing next; and (2) a delay indicator which specifies a time delay, Td. From these three items of information, the waveform playout system is instructed to begin playing the selected waveform starting at time Td after t0. In this typical example, the first three digitized samples of the selected waveform are w0, w1 and w2. At time t1, which equals time t0 plus Td, the waveform playout system produces an analog voltage level on the waveform output by converting the digitized sample, w0, to the corresponding voltage value. The reciprocal of the output sample rate, R, is a time Tout, which is the waveform output sample period. Time t2 is time t1 plus the output sample period Tout. At time t2 the waveform playout system produces an analog voltage level at the waveform output corresponding to digitized sample w1, etc. Continuing in this manner, the waveform playout system produces a time varying voltage on the waveform output line by converting the digitized samples of the selected waveform in sequence to the corresponding voltages at the output sample rate, R.
The time delay, Td, is provided because the mechanism by which the waveform playout system accesses the waveform memory requires time to adjust from one waveform to another after receiving the step indication from the sequencer. The time delay also allows the sequencer to align with greater resolution the time of playout change with the detailed timing behaviors of event inputs supplied to the sequencer.
As a result of the mechanism described above, the sequencer is able to instruct the waveform playout system to begin playout of the waveforms that the sequencer selects, such playouts beginning at times which the sequencer selects. In a typical AWG the sequencer may be controlled by a user-supplied program, and may engage in behaviors such as (1) selecting particular waveforms in a user-specified order, (2) repeating a waveform for a user-specified number of cycles, (3) making decisions based on the behavior of event inputs, (4) making decisions based on moments in time when the behaviors of the event inputs occur, and (5) various combinations of these behaviors, as is well known in the art and represented in commercially available products.
AWGs are used in the test and measurement environment to simulate a real life situation for testing various devices. For example, a test signal from an AWG, such as a wireless communication signal, may be used to test the operation of a cellular telephone. In a real environment the wireless communication signal may be interrupted by some form of interference, such as electrical or physical interference. Electrical interference may take the form of a radiated signal that swamps the wireless communication signal. Physical interference may take the form of a building or terrain that temporarily blocks the wireless communication signal. However, the wireless communication signal continues to transmit even though the signal is not being received. When the interference is removed, the received wireless communication signal has phase coherence with the wireless communication signal prior to the interference. FIG. 3 illustrates this where the signal generator provides the wireless communication signal, represented by a sine wave, and a zero level voltage source which represents interference. The switch determines when interference occurs, and the waveforms show how the sine signal continues during the period that the zero level is being transmitted so that, when the zero level is removed, the sine signal has phase coherence with the sine signal that existed prior to the “interference.”
When switching between waveforms in the waveform memory, current practice is for the AWG to start at the beginning of each new waveform, i.e., the waveform playout system always starts the waveform playout with digitized sample w0. However, sometimes the user wants the switching to be phase coherent to simulate a wireless communication signal, as discussed above, which means that the output waveforms should behave as though the waveform is being switched between generators that are synchronized with each other, i.e., the generators are always playing, rather than as though the switching operation resets the generator. FIG. 4 is a timing diagram illustrating the difference between a desired phase coherent waveform output and a waveform output according to the prior art.
What is desired is a phase coherent playback in an arbitrary waveform generator when switching between waveforms to a previously selected waveform.